Without getting too specific, the difference between a #5 and a #7.5 is in how long (in distance) it takes to transition from one route to another. Higher numbers take longer to transition. As a direct result, the higher number switch is actually much larger (in length) than the lower number.
Whether you choose a #5 or a #7.5 to make a crossover between your two loops depends on what type of equipment you run and how slow (or fast) you want to run through the crossover. If you run long equipment (engines and/or cars) and you want to perform high-speed crossovers, the #7.5 is almost mandatory. If you run all short equipment, high-speed crossovers would probably work with #5 switches.
Hope this helps.
Chuck
